Next homework is

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• Next homework is 7 due Friday at 1150 am last
  one before exam.
• Exam 2 is less than two weeks! Friday, November
  14th!
• Lets vote for exam style.
• Dont forget the Icko Iben Lecture on Wednesday.

2

• Want some extra credit?
• Download and print report form from course web
  site
• Attend the Iben Lecture on November 5th
• Obtain my signature before the lecture and answer
  the questions on form. Turn in by Nov. 14th
• Worth 12 points (1/2 a homework)

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Outline

• Finish up summary of star birth.
• Birth of a star onto the HR diagram.
• Stellar demise depends on the stellar mass.
• Higher mass stars live fast, die hard!
• The end of a 1 solar mass star
• Main sequence
• Red Giant
• Planetary nebula and white dwarf

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Other Things Besides Hydrogen in Molecular Clouds

• Molecules (e.g.)
• Carbon monoxide (CO)
• Water (H2O)
• Ammonia (NH3)
• Formaldehyde (H2CO)
• Ethyl alcohol (CH3CH2OH)
• Glycine (NH2CH2COOH)
• Acetic Acid (CH3COOH)
• Urea (NH2) 2 CO
• Dust particles
• Silicates, sometimes ice-coated
• Soot molecules

Polycyclic aromatic hydrocarbons (PAH)
Dust particle (interplanetary)
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Giant Molecular Clouds

• Cool lt 100 K
• Dense 102 105 H2 molecules/cm3
• Huge 10 100 pc across, 105 106 solar masses
• CO molecular emission dust emission trace
  structure

100 degrees
Infrared image from IRAS
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Low-Mass Star Formation - Summary
Young stellar object with bipolar outflow Age 5
x 105 yr
Giant molecular cloud
Dust-shrouded core Age 105 yr
Protoplanetary disk?
Main-sequence star Age 107 108 yr Hydrogen
fusion powered Creates emission or reflection
nebula Inhibits / stimulates further star form.
Magnetically active protostar (T Tauri star) Age
5 x 106 yr Gravitational collapse powered
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Movement onto the Main Sequence
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Main Sequence Mass Relation
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Main-Sequence Stars

• A.k.a. dwarf stars
• Hydrogen burning
• Hydrostatic equilibrium
• 91 of all nearby stars

Altair Type A8 V
Vega Type A0 V
Sun Type G2 V
Proxima Centauri Type M5 V
61 Cygni A Type K5 V
Regulus Type B3 V
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Stellar Middle Age
Stars like the Sun
Massive stars
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A Stars Demise Depends on Its Mass
Solar-mass main-sequence star
Helium-burning red giant
White dwarf and planetary nebula
10 MSun main-sequence star
Helium-burning red giant
Supergiant phases
Core-collapse supernova
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Movement off the Main Sequence
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Movement off the Main Sequence
Luminosity rate at which fuel is being
consumed Mass amount of fuel available
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Brown Dwarves M lt 0.08 Msun

• These are objects that are below 80 Jupiter
  masses.
• The central density and temperature do not get
  large enough for nuclear fusion to occur.
• These failed stars, gradually cool down and
  contract.
• Recently, there have been a number of discovered
  brown dwarves.

http//www.ast.cam.ac.uk/HST/press/gl229b.html
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Red Dwarves0.08 Msun lt M lt 0.4 Msun

• Fully convective interior, so helium produced in
  fusion gets evenly spread.
• The star turns all of its hydrogen to helium,
  then all fusion would stop.
• These stars live an incredibly long time
  hundreds of billions of years. As the Universe
  is thought to only be about 14 billion years old,
  none of these stars have yet made it to the end
  of their life.

http//www-astronomy.mps.ohio-state.edu/pogge/Ast
162/Unit2/RedDwarf.gif
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Evolutionary Path of a Solar-Mass Star
Planetary nebula
Helium flash
Asymptotic giant branch
Horizontal branch
Red giant
Protostar
Main sequence
White dwarf
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The Life of a 1 Solar Mass Star 0.4 MSun lt M
lt 4 MSun

• Example of how low mass stars will evolve on the
  HR Diagram http//rainman.astro.uiuc.edu/ddr/stel
  lar/archive/suntrackson.mpg

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Hydrostatic Equilibrium The Battle between
Gravity and Pressure

• Pressure pushes out and gravity pulls in an
  equilibrium
• This is why a main sequence star isnt shrinking
  even though its a big ball of gas.
• A stars life is all about this battle!

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What keeps it up?
The pressure comes from fusion. Gravity squeezes
hydrogen, until fusion starts. Then, the fusion
creates a back pressure.
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And when the Hydrogen Runs out?

• The low mass stars have radiative cores.
• First the hydrogen is burned in the core not hot
  enough to burn helium
• Then the core starts to shrink a little hydrogen
  shell burning (around the inert helium core)
  starts.
• This stops the collapse, and actually the outer
  envelope expands quickly.
• As the envelope expands, it cools so it becomes
  a Red Giant

Our Sun has about 5 billion more years left on
the main sequence.
http//www-astronomy.mps.ohio-state.edu/pogge/Ast
162/Unit2/LowerMS.gif
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The Interior of the Red Giant
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And then?

• So, we have a low mass star that has
• H fusing into He in the core
• Main sequence
• H fusing into He in a shell around the core
• Red giant (100 times larger, radius of 0.5 AU),
  turning the Earth to cinders
• What next? A Helium Flash!

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Helium Flash

• In the giant phase, the core temperature rises
• When temperature of the core reaches 100 million
  K, helium begins to fuse into carbon (C). Three
  Helium atoms fuse into Carbon and photons.
• The star gets bigger again
• Outer layers cool off
• .Helium burning happens suddenly and explosively

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Planetary Nebula Ejection

• Fusion slows down the helium has burned into
  carbon and oxygen, not enough pressure to fuse
  anything else.
• Stellar core collapses to high densities heats
  up
• The outer layers are pushed out by the hot
  radiation pressure of the core.
• The outer layers are almost all ejected
• The core (a white dwarf!) is made of ash from
  helium fusion carbon oxygen.

Outer layers blown off
Core collapses
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Planetary Nebulae
Hourglass Nebula
Ring Nebula
Cats Eye Nebula
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White Dwarfs and Planetary Nebulae

• Outer layers of the red giant star are blown away
  by radiation from the hot new white dwarf
• As they expand, they are lit from within by the
  white dwarf
• Distortions appear as expanding shell hits
  interstellar medium

T gt 200,000 K
NGC 2440
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White Dwarves!
http//oposite.stsci.edu/pubinfo/jpeg/M4WD.jpg
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What Keeps a White Dwarf up?

• The nuclear fusion stopped, and gravity begun to
  win the battle.
• Then, the electrons got so squashed together that
  they get pushed into degenerate states.
• Nearby electrons can not occupy the same energy
  states.
• This electron degeneracy causes pressure to
  counteract gravity

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Degeneracy Pressure

• Electrons are forced into higher energy levels
  than normal all of the lower levels are taken
• Effect manifests itself as pressure

NASA